1. Field of the Invention
The present invention relates to an exhaust gas purification device of an internal combustion engine.
2. Description of the Related Art
A particulate filter which carries an active oxygen release agent for releasing active oxygen and an NOx catalyst for purifying NOx has already been filed (Japanese Patent Application No. 2000-122409). An active oxygen release agent oxidizes and removes the particulate trapped on a particulate filter by active oxygen continuously in a short time. On the other hand, when the air-fuel ratio of the inflowing exhaust gas is lean, the catalyst absorbs the NOx in the exhaust gas, while when the air-fuel ratio of the inflowing exhaust gas becomes rich, it releases the absorbed NOx and reduces and purifies the NOx by the hydrocarbons in the exhaust gas.
Now, the particulate filter described in the above patent application continuously oxidizes a greater amount of particulate per unit time the higher the temperature (hereinafter referred to as xe2x80x9cthe filter temperaturexe2x80x9d). That is, the amount of the particulate which can be oxidized and removed continuously by the particulate filter described in the above patent application (hereinafter referred to as xe2x80x9cthe unit amount of continuous oxidation of particulatexe2x80x9d) becomes greater the higher the filter temperature. Therefore, there is a temperature at which the particulate in the inflowing exhaust gas can be continuously removed by oxidation in accordance with the unit amount of inflowing particulate for the particulate filter described in the above patent application (hereinafter referred to as xe2x80x9cthe continuous oxidation temperature of the particulatexe2x80x9d).
Therefore, the above patent application calls for the filter temperature to be controlled so that the filter temperature becomes higher than the particulate continuous oxidation temperature so that the unit amount of continuous oxidation of particulate becomes greater than the amount of particulate flowing into the particulate filter per unit time (hereinafter referred to as xe2x80x9cthe unit amount of inflowing particulatexe2x80x9d).
On the other hand, the NOx catalyst described in the above patent application absorbs the NOx once in advance while the air-fuel ratio of the inflowing exhaust gas is lean and releases, reduces, and purifies the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich. Therefore, to maintain the NOx purification rate of the NOx catalyst high as a whole, it is necessary to maintain the NOx absorption rate of the NOx catalyst high. Here, the NOx catalyst can absorb the NOx at a high absorption rate when its temperature is within a certain temperature range (hereinafter referred to as the xe2x80x9cNOx absorption temperature rangexe2x80x9d). Therefore, to maintain the NOx purification rate of the NOx catalyst high, it is necessary to maintain the filter temperature in the NOx absorption temperature range.
Therefore, the above patent application calls for the filter temperature to be controlled so that the temperature of the NOx catalyst is kept within the above NOx absorption temperature range when controlling the filter temperature so that the unit amount of continuous oxidation of the particulate becomes greater than the unit amount of inflowing particulate.
That is, the above patent application calls for the filter temperature to be controlled so that the filter temperature becomes higher than the particulate continuous oxidation temperature in the NOx absorption temperature range so as to maintain the particulate oxidation removal level and NOx purification level of the particulate filter at high levels.
Further, the above patent application calls for the temperature of the exhaust gas emitted from the internal combustion engine to be raised and the particulate filter to be heated by that high temperature exhaust gas so that the filter temperature becomes higher than the particulate continuous oxidation temperature in the NOx absorption temperature range when the filter temperature becomes lower than the low temperature of the NOx absorption temperature range or becomes lower than the particulate continuous oxidation temperature.
In the above patent application, however, when the NOx absorbed in the NOx catalyst should be reduced and purified, rich air-fuel ratio exhaust gas is supplied to the particulate filter, whereby NOx is made to be released from the NOx catalyst and the released NOx is reduced and purified by the hydrocarbons (HC) in the exhaust gas. The NOx catalyst described in the above patent application, however, can reduce and purify the NOx when its temperature is within a certain temperature range (hereinafter referred to as the xe2x80x9cNOx reduction temperature rangexe2x80x9d).
Therefore, to reduce the NOx with a high reduction rate, when supplying rich air-fuel ratio exhaust gas to the particulate filter, it is necessary that the filter temperature be kept in this NOx reduction temperature range. Further, even while reducing and purifying the NOx, a continuous oxidation and removal action of the particulate trapped on the particulate filter is preferable for maintaining the particulate oxidation removal level high.
In the particulate filter described in the above patent application, however, nothing at all is described regarding the point of control of the filter temperature during NOx reduction and purification. Therefore, there is room left for raising the particulate oxidation removal level and NOx purification level further even for the particulate filter described in the above patent application.
Therefore, an object of the present invention is to provide an exhaust gas purification device provided with particulate filters which can continuously oxidize and remove particulate and purify NOx where the particulate oxidation removal level and the NOx purification level are improved.
A first aspect of the invention for solving the above problems provides an exhaust gas purification device of an internal combustion engine provided with a pair of particulate filters arranged in parallel in an engine exhaust passage, an exhaust flow switching means for switching almost all of the engine exhaust gas emitted from an engine to cause it to flow into one of the particulate filters, and a combustion device for feeding to a particulate filter an exhaust gas having a higher temperature elevation capability than a temperature elevation capability of an engine exhaust gas able to cause the temperature of the particulate filters to rise, an NOx catalyst able to absorb NOx in the exhaust gas when an air-fuel ratio of the inflowing exhaust gas is lean and to release the absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich so as to enable the NOx to be reduced and purified by hydrocarbons in the exhaust gas and an oxidizing substance able to continuously oxidize and remove particulate being carried in the particulate filters, said NOx catalyst being able to release and reduce NOx when its temperature is in an NOx reduction temperature range, and said oxidizing substance being able to continuously oxidize and remove almost all inflowing particulate when the temperature of said oxidizing substance is higher than a particulate continuous oxidation temperature, characterized by being provided with a reduction judging means for judging if the NOx absorbed in the NOx catalyst should be reduced, suppressing by said exhaust flow switching means the inflow of engine exhaust gas to the one of the particulate filters carrying the NOx catalyst for which it is judged the NOx absorbed in the NOx catalyst should be reduced when it is judged by said reduction judging means that the NOx absorbed in the NC should be reduced, and feeding rich air-fuel ratio exhaust gas to said particulate filter from a combustion device so that the temperature of said particulate filter is maintained higher the particulate continuous oxidation temperature in a NOx reduction temperature range. That is, according to the present invention, rich air-fuel ratio is fed from the combustion device to a particulate filter in a state where the amount of the engine exhaust gas flowing into the particulate filter is reduced so that the temperature of the particulate filter is maintained higher than the particulate continuous oxidation temperature in the NOx reduction temperature range.
A second aspect of the invention provides the first aspect of the invention characterized by suppressing the inflow of engine exhaust gas to said particulate filter by said exhaust flow switching means when it is judged by said reduction judging means the NOx absorbed in the NOx catalyst should be reduced and when the temperature of the one of the particulate filters carrying the NOx catalyst for which it is judged that said NOx should be reduced is lower than a low temperature of the NOx reduction temperature range, feeding a rich air-fuel ratio exhaust gas having a concentration of oxygen higher than a predetermined concentration from said combustion device to said particulate filter to raise the temperature of said particulate filter to within the NOx reduction temperature range when the temperature of said combustion device is lower than said predetermined concentration, and feeding to said particulate filter a rich air-fuel ratio exhaust gas having a concentration of oxygen lower than said predetermined concentration from said combustion device when the temperature of said particulate filter becomes in the NOx reduction temperature range. According to this, rich air-fuel ratio exhaust gas for reducing the NOx is fed to the particulate filter only when the temperature of the particulate filter is in the NOx reduction temperature range.
A third aspect of the invention provides the first aspect of the invention characterized, when it is judged by said reduction judging means that the NOx absorbed in the NOx catalyst should be reduced, by suppressing the inflow of engine exhaust gas to the one of the particulate filters provided with the NOx for which it is judged said NOx should be reduced and feeding lean air-fuel ratio exhaust gas from said combustion device to said particulate filter to make the temperature of said particulate filter rise to within the NOx reduction temperature range when the temperature of said one particulate filter is lower than the low temperature of the NOx reduction temperature range and feeding rich air-fuel ratio exhaust gas from said combustion device to said particulate filter when the temperature of said particulate filter becomes within the NOx reduction temperature range. According to this, rich air-fuel ratio exhaust gas for reducing the NOx is fed to a particulate filter only when the temperature of the particulate filter is in the NOx reduction temperature range.
A fourth aspect of the invention provides the first aspect of the invention characterized by feeding lean air-fuel ratio exhaust gas from said combustion device to said particulate filter when the reduction of the NOx absorbed in the NOx catalyst is completed. According to this, when the reduction of the NOx absorbed in the NOx catalyst is completed, the hydrocarbons deposited on a particulate filter are burned off by the lean air-fuel ratio exhaust gas fed from the combustion device to the particulate filter.
A fifth aspect of the invention provides one of the first to fourth aspects of the invention characterized by being provided with an air-fuel ratio sensor able to detect the air-fuel ratio of the exhaust gas flowing out from a particulate filter and controlling the air-fuel ratio of the exhaust gas being fed from said combustion device to said particulate filter based on the output of said air-fuel ratio sensor.
A sixth aspect of the invention provides the first aspect of the invention characterized by suppressing the inflow of engine exhaust gas to one of the particulate filters by said exhaust flow switching means at the time of engine startup and feeding exhaust gas from said combustion device to said one particulate filter to make the temperature of said one particulate filter rise. According to this, exhaust gas is fed from the combustion device to a particulate filter in a state where the amount of the engine exhaust gas flowing into the particulate filter is reduced.
A seventh aspect of the invention provides the sixth aspect of the invention characterized by being provided with an engine operation controlling means for selectively executing a first engine operation where the engine operation is controlled based on a required torque and engine speed and a second engine operation where an engine exhaust gas having a temperature higher than the temperature of the engine exhaust gas emitted from the engine when said first engine operation is executed is made to be emitted from the engine and by executing second engine operation control by said engine operation controlling means to make the temperature of the other particulate filter rise at the time of engine startup. That is, according to this, one of the particulate filters is fed with exhaust gas from the combustion device, while the other of the particulate filters is supplied with engine exhaust gas from the engine.
An eighth aspect of the invention provides the first aspect of the invention characterized by having said NOx catalyst able to absorb a sulfur ingredient in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and to release the absorbed sulfur ingredient when the air-fuel ratio of the inflowing exhaust gas becomes rich and its temperature is higher than a sulfur ingredient release temperature where the absorbed sulfur ingredient can be released; being provided with a sulfur ingredient release judging means for judging whether the sulfur ingredient absorbed in the NOx catalyst should be released from the NOx catalyst; causing the temperature of said particulate filter to rise to a sulfur ingredient release temperature when it is judged by said sulfur ingredient release judging means that the sulfur ingredient absorbed in the NOx catalyst should be released and the temperature of the one of the particulate filters for which it is judged that the sulfur ingredient should be released is lower than a sulfur ingredient release temperature and suppressing the inflow of engine exhaust gas to said particulate filter by said exhaust flow switching means and feeding rich air-fuel ratio exhaust gas from said combustion device to said particulate filter when the temperature of said particulate filter reaches the sulfur ingredient release temperature.
A ninth aspect of the invention provides the first aspect of the invention characterized by having said NOx catalyst able to absorb a sulfur ingredient in the exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and to release the absorbed sulfur ingredient when the air-fuel ratio of the inflowing exhaust gas becomes rich and its temperature is higher than a sulfur ingredient release temperature where the absorbed sulfur ingredient can be released; being provided with a sulfur ingredient release judging means for judging whether the sulfur ingredient absorbed in the NOx catalyst should be released from the NOx catalyst; causing the temperature of said particulate filter to rise to a sulfur ingredient release temperature when it is judged by said sulfur ingredient release judging means that the sulfur ingredient absorbed in the NOx catalyst should be released and the temperature of the one of the particulate filters for which it is judged that the sulfur ingredient should be released is lower than a sulfur ingredient release temperature and suppressing the inflow of engine exhaust gas to said particulate filter by said exhaust flow switching means and alternately feeding rich air-fuel ratio exhaust gas and lean air-fuel ratio exhaust gas from said combustion device to said particulate filter when the temperature of said particulate filter reaches the sulfur ingredient release temperature. According to this, after the temperature of a particulate filter reaches the sulfur ingredient release temperature, the sulfur ingredient is released from the NOx catalyst while the rise of that temperature is suppressed.
A 10th aspect of the invention provides the ninth aspect of the invention characterized by maintaining the air-fuel ratio of the exhaust gas fed from said combustion device to the particulate filter lean when the sulfur ingredient is being released from the NOx catalyst and the engine speed becomes smaller than a predetermined value.
An 11th aspect of the invention provides the eighth or ninth aspect of the invention characterized by being provided with an engine operation controlling means for selectively executing a first engine operation where engine operation is controlled based on a required torque and engine speed and a second engine operation for making the engine emit an engine exhaust gas having a temperature higher than the temperature of the engine exhaust gas emitted from the engine when said first engine operation is executed and by executing the second engine operation by said engine operation controlling means so as to make the temperature of the one of the particulate filters for which it is judged that said sulfur ingredient should be released rise to the sulfur ingredient release temperature when it is judged by said sulfur ingredient release judging means that the sulfur ingredient absorbed in the NOx catalyst should be released.
A 12th aspect of the invention provides any one of the first to 11th aspects of the invention characterized by having said combustion device take in engine exhaust gas and cause the fuel to burn by the oxygen in said engine exhaust gas.
A 13th aspect of the invention provides the 12th aspect of the invention characterized by having an air feeding means for feeding air into the engine exhaust gas which said combustion device takes in.
A 14th aspect of the invention provides any one of the first to 11th aspects of the invention characterized by having said combustion device take in air and take in exhaust gas.
A 15th aspect of the invention provides the 14th aspect of the invention characterized by having said combustion device take in exhaust gas flowing out from a particulate filter.
A 16th aspect of the invention provides any one of the first to 11th aspects of the invention characterized by being provided with outflow suppressing means for suppressing the outflow of exhaust gas from downstream of a particulate filter and by suppressing the output of exhaust gas to downstream of one of the particulate filters by said outflow suppressing means when suppressing the inflow of engine exhaust gas to said one of the particulate filters by said exhaust flow switching means.
A 17th aspect of the invention provides the 16th aspect of the invention characterized by being provided with a pressure sensor for detecting a pressure in a particulate filter, detecting by a pressure sensor the pressure in one of the particulate filters when suppressing the inflow of engine exhaust gas to said one of the particulate filters by said exhaust flow switching means and suppressing the outflow of exhaust gas from downstream of said one of the particulate filters by said outflow suppressing means, and controlling the degree of suppression of outflow by said outflow suppressing means so that the pressure in said one of the particulate filters does not exceed a pressure so high as to inhibit operation of the combustion device.
An 18th aspect of the invention provides the first aspect of the invention characterized by separately extending engine exhaust passages of a downstream side of the particulate filters over at least predetermined distances from said particulate filters and setting said predetermined distances to distances where the engine exhaust gas flowing out from the other of the particulate filters will not flow to one of the particulate filters from downstream when suppressing the inflow of engine exhaust gas to one of the particulate filters by said exhaust flow switching means so as to cause almost all of the engine exhaust gas to flow into the other of the particulate filters.
The 19th aspect of the invention provides the first aspect of the invention characterized in that a check valve for preventing back flow of an exhaust gas from a particulate filter to the combustion device is arranged in an exhaust feed passage feeding exhaust gas from said combustion device to the particulate filter.